Three-dimensional semiconductor template for making high efficiency thin-film solar cells

ABSTRACT

A semiconductor template having a top surface aligned along a (100) crystallographic orientation plane and an inverted pyramidal cavity defined by a plurality of walls aligned along a (111) crystallographic orientation plane. A method for manufacturing a semiconductor template by selectively removing silicon material from a silicon template to form a top surface aligned along a (100) crystallographic plane of the silicon template and a plurality of walls defining an inverted pyramidal cavity each aligned along a (111) crystallographic plane of the silicon template.

This application claims the benefit of provisional patent application61/114,378 filed on Nov. 13, 2008, which is hereby incorporated byreference.

FIELD

This disclosure relates in general to the field of photovoltaics andsolar cells, and more particularly to semiconductor templates andmethods for making semiconductor templates for use in manufacturingthree-dimensional thin-film solar cells.

DESCRIPTION OF THE RELATED ART

Current methods for manufacturing a three-dimensional thin-film solarcell (3-D TFSC) include forming a 3-Dimensional thin-film siliconsubstrate (3-D TFSS) using a silicon template. The template may comprisea plurality of posts and a plurality of trenches between said aplurality of posts. The 3-D TFSS may then be formed by forming asacrificial layer on the template, subsequently depositing asemiconductor layer, selective etching the sacrificial layer andreleasing the semiconductor layer from the template. More specifically,the said semiconductor layer is a self-supporting, free-standingthree-dimensional (3D) epitaxial silicon thin film deposited on andreleased from a low-cost reusable crystalline silicon substratetemplate. The reusable silicon template may be reused to form the 3Dfilm numerous times before being reconditioned or recycled. Selectportions of the released 3-D TFSS are then doped with a first dopant,and other select portions are than doped with a second dopant. Aftersurface passivation processes, emitter and base metallization regionsare formed to complete the solar cell structure. FIG. 1A illustrates apartial view of a re-usable mono-crystalline silicon template withhexagonal-prism posts according to the U.S. Patent Pub. No.2008/0264477A1. FIG. 1B illustrates a partial view of a 3D thin-film,hexagonal-honeycomb-prism substrate with rear/bottom base silicon layerafter release from the reusable template according to the U.S. PatentPub. No. 2008/0264477A1.

The above referenced three-dimensional thin film solar cell templates,substrates, and cells provide cost, performance, and mechanical strengthadvantages compared to traditional flat solar cells with a similaramount of silicon because 3-D TFSC have superior mechanical strength,better light trapping, and lower cell processing costs because of theirself-aligned nature.

From a mechanical structure perspective, given a fixed amount of siliconstructural material, a honeycomb 3-D TFSS may provide a desirablemechanical rigidity and strength. However, from the fabrication processperspective, the trenches among the neighboring hexagonal pillars on thetemplate need to be filled by epitaxial silicon growth and the substrateformed by the filled layer needs to be released from the template. Theseprocesses are often costly and difficult. Design and processimprovements need to be made in making the relatively high aspect ratiostrenches, epitaxial filling of the trenches and releasing a TFSS fromthe trenches.

Additionally, known flat thin film solar cells often require surfacetexturing to reduce reflectance losses which requires a minimum filmthickness of preferably tens of microns (e.g., >30 μm) to avoidtexturing etch-induced punch-through pinholes. Also, flat thin-filmsilicon substrates may have reduced mean optical path length whichreduces IR absorption and results in reduced cell quantum efficiency.And flat thin-film crystalline silicon substrates may have poormechanical strength for cell and module processing needs. Micro crackingdefects at substrate edges and pinholes defects within the substratecould cause cracking initiations and these cracks propagate easily alongthe crystallographic directions.

SUMMARY

Therefore a need has arisen for a template which provides fabricationprocess improvements and manufacturing costs reductions for forming athree-dimensional thin-film solar cell substrate (3-D TFSC substrate).In accordance with the disclosed subject matter, a three-dimensionalsemiconductor template is provided which substantially eliminates orreduces disadvantages and problems associated with previously developedsilicon substrates.

According to one aspect of the disclosed subject matter, athree-dimensional (3-D) semiconductor template is provided having a topsurface aligned along a (100) crystallographic orientation plane of thetemplate and an inverted pyramidal cavity formed by walls each alignedalong a (111) crystallographic orientation plane of the template. In oneembodiment, the template is made of mono-crystalline silicon. In yetanother embodiment, the template has a multiple inverted pyramidalcavities which may be arranged in arrays or in a staggered pattern alongthe template surface. Further, the inverted pyramidal cavities may bedifferent sizes and pyramidal shapes.

Additionally, fabrication methods for forming a three-dimensionalsilicon template are provided. In one embodiment, silicon material isselectively removed to form walls aligned along a (111) crystallographicplane of the silicon template defining the inverted pyramidal cavity.The removal may be by anisotropic etching of the silicon template. Theinverted pyramidal silicon template may be made by anisotropic siliconetching using a photolithographically defined hard masking layer, suchas silicon dioxide.

Technical advantages of the disclosed subject matter include fabricationprocess improvements and manufacturing cost reductions by utilizing the(111) crystallographic orientation plane to make inverted pyramidcavities on the template. Further, the inverted pyramidal cavitiesprovide increased mechanical rigidity to the 3-D TFSS that is made fromthe template.

A technical advantage of the simplified fabrication processes and highergas-to-silicon conversion ratio of epitaxial growth provided when usinga template having inverted pyramidal cavities is an inverted pyramidalcavity based 3-D TFSS provides improved mechanical rigidity andstrength. The strength of the template may be adjusted according to thearrays and staggered patterns of inverted pyramidal cavities provided.

A technical advantage of the present disclosure is innovative solar celldesigns and technologies based on the use of self-supporting,free-standing, three-dimensional (3D) silicon thin films. The 3-D TFSCsdescribed may be made to be relatively rigid, semi-rigid, or flexibledepending on the structural design parameters of the cell substrate.Given an equal amount of silicon usage, the 3-D TFSS disclosed providesadvantages over substrates made of flat thin-film (TF) crystallinesilicon such as the following:

(1) Three-dimensional thin-film solar cells disclosed do not require aminimum film thickness;

(2) Three-dimensional thin-film solar cells disclosed trap lightextremely efficiently by virtue of their 3D nature;

(3) Three-dimensional thin-film solar cells disclosed are mechanicallyrobust because of their unique 3D structure, providing enhancedmechanical strength and handle-ability.

Further technical advantages of the disclosed subject matter include: 1)the semiconductor templates consist of known crystallographic siliconplanes, i.e., the (111) and (100) planes and the epitaxial silicon layergrown from these two silicon planes yields better quality than from DRIEetched silicon 3-D surfaces, and 2) the large cavity opening angle)(70.6° of the disclosed silicon template formed by the cavity sidewall(111) planes is much wider than that may be etched from using deepreactive ion etch (DRIE) silicon etch. Therefore, the porous siliconformation, epitaxial silicon growth, and releasing of 3-D TFSS are morepractical and cost efficient than a DRIE etched template.

The disclosed subject matter, as well as additional novel features, willbe apparent from the description provided herein. The intent of thissummary is not to be a comprehensive description of the claimed subjectmatter, but rather to provide a short overview of some of the subjectmatter's functionality. Other systems, methods, features and advantageshere provided will become apparent to one with skill in the art uponexamination of the following FIGURES and detailed description. It isintended that all such additional systems, methods, features andadvantages included within this description, be within the scope of theaccompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

For a more complete understanding of the disclosed subject matter andadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numbers indicate like features and wherein:

FIG. 1A (PRIOR ART) shows a prior art mono-crystalline silicon template;

FIG. 1B (PRIOR ART) shows a prior art three-dimensional thin-filmsubstrate after release from the template in FIG. 1A;

FIG. 2 illustrates a cross-sectional drawing of an inverted pyramidaltemplate and a released corresponding three-dimensional thin-filmsilicon substrate;

FIG. 3 is an image of a fabricated inverted pyramidal silicon template;

FIGS. 4A, 4B, and 4C are images of a fabricated three-dimensionalthin-film silicon substrate;

FIG. 5 is a process flow depicting major fabrication process steps formanufacturing an inverted pyramidal silicon template andthree-dimensional thin-film silicon substrate;

FIGS. 6A through 6G illustrates a process flow for manufacturing aninverted pyramidal silicon template and a three-dimensional thin-filmsilicon substrate;

FIG. 7 illustrates an array inverted pyramidal pattern on asemiconductor template;

FIGS. 8A through 8D illustrate alternative staggered inverted pyramidallayout patterns on a semiconductor template;

FIG. 9 is a process flow depicting major fabrication process steps formanufacturing a three-dimensional thin-film solar cell; and

FIGS. 10A through 10D illustrate a process flow for manufacturing athree-dimensional thin-film solar cell.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The following description is not to be taken in a limiting sense, but ismade for the purpose of describing the general principles of the presentdisclosure. The scope of the present disclosure should be determinedwith reference to the claims. Exemplary embodiments of the presentdisclosure are illustrated in the drawings, like numbers being used torefer to like and corresponding parts of the various drawings.

FIG. 1A illustrates a partial view of a re-usable mono-crystallinesilicon template with hexagonal-prism posts disclosed in U.S. Pat. Pub.No. 2008/0264477A1. The hexagonal pillars are etched by deep-reactiveion etching (DRIE) with photolithographically patterned photoresist asthe hard masking layer. The DRIE etching provides well definedhigh-aspect ratio gaps between the pillars, however the narrow gaps aredifficult to fill by the epitaxial silicon growth and it is difficult torelease the epitaxial layer from such a template.

FIG. 1B illustrates a partial view of a 3D thin-filmhexagonal-honeycomb-prism substrate with a rear/bottom base siliconlayer after release from a reusable template disclosed in U.S. Pat. Pub.No. 2008/0264477A1.

FIG. 2 illustrates a cross-sectional drawing of an inverted pyramidalsemiconductor template and a corresponding released three-dimensionalthin-film semiconductor substrate. Inverted pyramidal template 2 is usedto form corresponding released 3-D TFSS 8. As shown, the invertedpyramidal template consists of large inverted pyramidal cavities 3 andsmall inverted pyramidal cavities 4. Large inverted pyramidal cavity 3and small inverted pyramidal cavity 4 which are defined by walls, suchas wall 5 aligned along a (111) plane of inverted pyramidal template 2.Top surface 6 is aligned along a (100) plane of inverted pyramidaltemplate 2. The inverted pyramid-shape cavities may be chemically etchedby anisotropic silicon etchant and the angle between a sidewall (such aswall 5) and a top lateral plane (such as top surface 6), is about 54.7°(angle 7)—which is the defined angle between two (111) and (100) siliconcrystallographic planes. Shown, small inverted pyramidal cavities 4 havean apex defined by walls aligned along the (111) crystallographic planesgiving the apex, or tip, of the inverted pyramid an angle of 70.6°.Large inverted pyramidal cavities 3 have a flat apex, or flat tip,aligned along the (100) crystallographic plane. A semiconductor templateof the disclosed subject matter may employ various apex styles dependenton shape of the 3-D TFSS desired.

Released 3-D TFSS 8 has a bottom surface profile conformal to the top ofinverted pyramidal template 2. Wall 9 defines inverted pyramidal cavity11 on released 3-D TFSS 8 and surface ridge 10 defines the base openingof inverted pyramidal cavity 11.

FIG. 3 presents a top Scanning Electron Microscope (SEM) partial view ofa fabricated inverted pyramidal silicon template. Structured silicontemplate 20 consists of anisotropically etched large pyramid cavity 21and small pyramid cavity 24. The top opening size 22 of the largecavities is in the range of 10 um to 1 mm and the top opening size 25 ofthe small cavities is a partial of 22. As shown the opening size of thelarge cavities, 22, is about 300 um and opening size of the smallcavities, 25, is about 150 um. In this case, the depth of the smallcavities is about 110 um and the depth of the large cavities is about200 um. Top cavity surface 26 and bottom cavity surface 28 are alignedalong (100) silicon crystallographic planes and sidewalls 29 of thecavities are aligned along (111) silicon crystallographic planes. Smallpyramid cavity 24 has a pointed cavity apex/bottom which occurs at theintersection of four (111) sidewalls. Large pyramid cavity bottom 28 hasa flat cavity bottom/apex the size of which may be in the range of 0 to100 um. As shown, top cavity surface 26 is the ridge defining theopening between inverted pyramidal cavities. Top cavity surface 26 ispreferably narrow and less than 10 um wide.

FIG. 4A presents a SEM partial view 40 of an embodiment of a fabricatedthree-dimensional thin-film silicon substrate from a tilted topperspective. The 3-D TFSS consists of a staggered pattern of largecavities, 42, and small cavities, 44. However due to the nature of theepitaxial silicon growth, crystallographic faceting occurs and resultsin a structure quite different from the inverted pyramidal cavities onthe semiconductor template from which the 3-D TFSS was made inaccordance with the disclosed subject matter. Further, ridge 46 betweenthe cavities and defining a base opening of large cavity 42 is wider onits top side due to epitaxial overgrowth on the top surfaces alignedalong the (100) crystallographic plane of a semiconductor template. Thedegree or amount of the shape change depends on the overall epitaxialsilicon thickness. A thicker epitaxial thickness results in more shapechange from the original silicon template structure from which 3-D TFSSwas made. These shape and geometrical changes improve the mechanicalstrength of the 3-D TFSS and make the light trapping more effective.

FIG. 4B presents a SEM partial view 60 of an embodiment of a fabricated3-D TFSS from a tilted bottom perspective. The shown 3-D TFSS backsideis the reverse of the silicon template structure from which 3-D TFSS wasmade, which consisted of inverted pyramidal cavities forming largepyramid 62 and small pyramid 64 and ridges 66 defining the base openingsof the inverted pyramidal cavities on the 3-D TFSS.

FIG. 4C presents a SEM partial view 80 of an embodiment of a fabricated3-D TFSS from a cross-sectional perspective. Depending on the epitaxialgrowth process conditions, the thickness of the top ridge 82, sidewall84 and bottom 86 may be different and could be purposefully tuned toachieve optimum mechanical, optical and electrical performances.

A 3-D TFSC fabrication process in accordance with the disclosed subjectmatter may comprise the following major steps:

(1) Template fabrication: 3-D inverted pyramidal patterns/structures areetched from a silicon wafer front surface and into the bulk silicon toform a silicon template. The structured silicon template is then used inthe formation of a 3-D TFSS. The template may be capable of being usednumerous times to fabricate numerous 3-D TFSS before being reconditionedor recycled. The template may be reused for as long as it remainsrelatively free of dislocations and/or for as long as it maintains anacceptable pyramid pattern having a pyramidal size and sidewall surfaceconditions within acceptable control limits (e.g. as gauged by in-linemetrology);

(2) 3-D TFSS (substrate) fabrication: After forming low and highporosity porous silicon layers (or a graded porosity porous siliconlayer) on the template front surface, epitaxial silicon is grown fromthe front template surface. In-situ emitter and base doping may beconducted during epitaxial silicon growth. The 3-D TFSS is then formedby releasing/separating the epitaxial silicon layer from the template.The released 3-D TFSS goes through subsequent solar cell processingsteps and the template may be re-used after proper cleaning and optionalreconditioning;

(3) Cell fabrication process: In the process module, the 3-D TFSS isoptionally doped to form emitter and base regions if the doping is notconducted prior to the epitaxial layer release. Then a surfacepassivation layer and an antireflection layer are deposited, optionalcontact openings are formed, and metallization steps are conducted toform a 3-D TFSC; and

(4) Module assembly and integration process: After proper testing andinspection, the fabricated 3-D TFSC may then optionally be mechanicallyreinforced, interconnected, encapsulated and mounted in the panels.

FIG. 5 is an embodiment of a process flow depicting major fabricationprocess steps for manufacturing an inverted pyramidal silicon templateand three-dimensional thin-film silicon substrate. The silicon templatemaking process starts with step 105 beginning with a mono-crystalline(100) silicon wafer. The starting wafer may be in circular or squareshapes. Step 110 involves forming a thin hard masking layer on theexposed wafer surfaces. The hard masking layer is used to mask thesilicon surface areas that do not need to be etched in the latersteps—the surface areas that will become the top surface of thetemplate. The proper hard masking layer includes, but is not limited to,thermally grown silicon oxide and low-pressure vapor phase deposited(LPCVD) silicon nitride. Step 115 involves a photolithography step,which consists of photoresist coating, baking, UV light exposure over aphotomask, post baking, photoresist developing, wafer cleaning anddrying. After this step, the pattern on the photomask depicting an arrayor a staggered pattern of inverted pyramidal base openings, will betransferred to the photoresist layer. The patterned photoresist layer isused as a soft masking layer for the hard masking layer etching of step120. Step 120 involves further transferring the photoresist pattern tothe hard masking layer layered underneath by chemical etching, such asetching a thin silicon oxide layer with buffered HF solution. Other wetetching methods and dry etching methods as known in semiconductor andMEMS wafer processing may also be used. In step 125 the remaining softmasking layer, i.e. the photoresist layer, is removed and the wafer iscleaned. Examples of photoresist removal process include wet methods,such as using acetone or piranha solution (a mixture of sulfuric acidand hydrogen peroxide), or dry methods such as oxygen plasma ashing. Instep 130 the wafers are batch loaded in an anisotropic silicon wetetchant such as KOH solution. The typical etch temperature is in therange of 50° C. to 80° C. and etch rate is about 0.2 um/min to 1 um/min.TMAH (tetramethylammonium hydroxide) is an alternative anisotropicsilicon etching chemical. The KOH or TMAH silicon etch rate depends uponthe orientations to crystalline silicon planes. The (111) family ofcrystallographic planes are etched at a very slow rate and are normally“stop” planes for the anisotropic etching of a (100) silicon wafer withpatterned hard mask. As a result, the intersection of two (111) planesor a (111) plane with a bottom (100) plane produce anisotropic etchingstructures for (100) silicon wafers after a time-controlled etch.Examples of these structures include V-grooves and pyramidal cavitieswith sharp tip cavity bottom (where (111) planes meet) or a small flatcavity bottom (a remaining (100) plane).

Advantages of the inverted pyramidal template of the present disclosureinclude: (i) the template KOH etching process is more convenient tocontrol and has a low manufacturing cost compared to other siliconetching methods, such as RIE dry etching; (ii) the (111) plane dominant3-D structure provides good porous silicon forming uniformity andepitaxial silicon quality due to the (111) plane sidewall smoothness andpredictable and repeatable epitaxial growth rates; (iii) the dimensions,shape, and profiles of the inverted pyramidal structure on the templatemay be maintained conveniently and restored easily by a short KOHetching if needed after multiple template reuse cycles.

In step 135 of FIG. 5 the remaining hard masking layer is removed, by HFsolution in the case the hard masking layer is silicon dioxide. Next,the wafer may be cleaned in standard SC1 (mixture of NH₄OH and H₂O₂) andSC2 (mixture of HCL and H₂O₂) wafer wet cleaning solutions followed by athorough deionized wafer rinsing and hot N₂ drying. The disclosedprocess results in a silicon template with inverted pyramidal cavities.

Step 140 of FIG. 5 marks the beginning of a silicon template re-usecycle. In step 145, a porous silicon layer is formed by electrochemicalHF etching on the silicon template front surfaces. The porous siliconlayer is to be used as a sacrificial layer for epitaxial silicon layerrelease. The porous silicon layer preferably consists of two thin layerswith different porosities. The first thin porous silicon layer is a toplayer and is formed first from the bulk silicon wafer. The first thinlayer preferably has a lower porosity of 10%˜35%. The second thin poroussilicon layer is directly grown from the bulk silicon and is underneaththe first thin layer of porous silicon. The 2^(nd) thin porous siliconlayer preferably has a higher porosity in the range of 40%˜80%. The topporous silicon layer is used as a crystalline seed layer for highquality epitaxial silicon growth and the bottom underneath higherporosity porous silicon layer is used for facilitating TFSS release dueto its less dense physical connections between the epitaxial and bulksilicon interfaces and its weak mechanical strength. Alternatively, asingle porous silicon layer with a progressively increased or gradedporosity from top to bottom may also be used. In this case, the topportion of the porous silicon layer has a low porosity of 10% to 35% andthe lower portion of the porous silicon layer has a high porosity of 40%to 80%. In step 150, before the epitaxial silicon growth, the wafer maybe baked in a high temperature (at 950° C. to 1150° C.) hydrogenenvironment within the epitaxial silicon deposition reactor in order toform coalesced structures with relatively large voids within thehigher-porosity porous silicon layer (or portion of a single layer)while forming a continuous surface seed layer of crystalline silicon onthe lower-porosity porous silicon layer (or portion of a single layer).In step 155, a mono-crystalline silicon epitaxial layer is deposited onthe front side only. The bulk base of the epitaxial layer is p-type,boron (B₂H₆) doped. The thickness of the epitaxial layer is preferablyin the range of 5 um to 60 um. In step 160, prior to the release of theepitaxial silicon layer, an encompassing border trench may be made onthe peripheral of the active wafer area to facilitate the release of theTFSS. The encompassing trenches may be formed by controlled lasercutting and their depths are preferably in the range of 5 um to 100 um.The trenches define the boundary of the 3-D TFSS to be released andallow initiation of the release from the trenched region. The remainingepitaxial silicon layer may be removed by mechanical grinding orpolishing of the template edges. In step 165, the epitaxial layer ofsilicon is released and separated from the silicon template. Thereleased epitaxial silicon layer is referred to as a 3-D thin filmsilicon substrate (3-D TFSS). The epitaxial layer release methodsdisclosed in U.S. patent application Ser. No. 12/473,811 entitled,SUBSTRATE RELEASE METHODS AND APPARATUS are hereby incorporated byreference. The 3-D TFSS may be released in an ultrasonic DI-water bath.Or in another release method, the 3-D TFSS may be released by directpulling with wafer backside and top epitaxial vacuum chucked. In anotherrelease method, the epitaxial layer is released by direct pulling withwafer backside and top epitaxial vacuum chucked. Using this method theporous silicon layer may be fully or partially fractured. The chucks mayuse either electrostatic or vacuum chucking to secure the wafer. Thewafer is first placed on bottom wafer chuck with TFSS substrate facingupwards. A bottom chuck secures the template side of wafer, and the topwafer chuck is gently lowered and secures TFSS substrate side of thewafer. The activated pulling mechanism lifts top chuck upwards, and themovement may be guided evenly by slider rails.

In step 170, the released 3-D TFSS backside surface is cleaned by shortsilicon etching using KOH or TMAH solutions to remove the silicon debrisand fully or partially remove the quasi-mono-crystalline silicon (QMS)layer. After removal of the epitaxial silicon layer from the template,the template is cleaned in step 175 by using diluted HF and diluted wetsilicon etch solution, such as TMAH and/or KOH to remove the remainingporous silicon layers and silicon particles. Then the template isfurther cleaned by conventional silicon wafer cleaning methods, such asSC1 and SC2 wet cleaning to removal possible organic and metalliccontaminations. Finally, after proper rinsing with DI water and N₂drying, the template is ready for another re-use cycle.

FIGS. 6A through 6D depict cross-sectional drawings illustrating aprocess flow for manufacturing an inverted pyramidal silicon template.

FIG. 6A illustrates mono-crystalline (100) silicon wafer 201 after ahard mask deposition and soft mask patterning. The thickness of siliconwafer 201 is in the range of 0.5 mm to 2 mm. The disclosed templateforming process may be applied to a polished or non-polished surface.Alternatively, silicon wafers with square or quasi-square shapes mayalso be used. Front hard mask layer 202 and backside hard mask layer 203are thermally grown silicon oxide layers having a thickness in the rangeof 0.5 um to 1.5 um. The oxide on the wafer edge is not shown.Photolithographic defined or screen-printed photoresist pattern 204 isapplied on the front wafer surface. The photolithography processincludes photoresist coating, baking, exposure, developing and postbaking. The photoresist pattern consists of staggered pattern of largeinverted pyramidal base opening 205 and small inverted pyramidal baseopening 206. However, the photoresist pattern may also be an array ofequally sized inverted pyramidal base openings. The inverted pyramidalbase opening patterns should be precisely aligned to the wafer <100>direction on the front surface.

FIG. 6B illustrates wafer 212 after the inverted pyramidal base openingpattern is transferred to the hard masking layer, front oxide layer 214.The pattern transferring from the photoresist layer to the oxide layeris achieved by controlled oxide etching in a buffered HF solution.During HF wet etching, backside and edge oxide layer 216 is protectedand keeps an original thickness. The oxide pattern on the front side ofwafer 212 then consists of a staggered pattern of large invertedpyramidal base opening 217 and small inverted pyramidal base opening 218that are aligned to the <100> crystallographic directions on the frontlateral plane. After the pattern transfer, the remaining photoresistlayer is removed by wet or dry photoresist removal methods. Therefore,the photoresist layer is not shown in FIG. 6B.

FIG. 6C illustrates wafer 220 after a silicon anisotropic etching step.Large inverted pyramidal cavity 227 and small inverted pyramidal cavity228 are formed after a timed-controlled silicon etch in a KOH or TMAHsolution. The etching temperature is preferably in the range of 50° C.to 80° C. During the silicon etching, the wafer backside and edgesurfaces are fully protected by un-patterned oxide layer 226. The KOHetch may be timely controlled so that a certain inverted pyramidalcavity depth may be reached. Alternatively, the KOH etching may beself-terminated when the (111) walls forming the inverted pyramidalcavity meet at the cavity bottom/apex. After the KOH etching, remainingoxide layers 224 and 226 are thinner than before the etching because theoxide is also etched in the KOH or TMAH solution, but with a much sloweretch rate than the silicon etch.

FIG. 6D illustrates inverted pyramidal silicon template 232 afterremoving the remaining oxide layer in a diluted HF solution followed bystandard wafer cleaning in SC2 and SC2, DI water rinsing, and N₂ drying.The width of the ridges forming the base openings of the invertedpyramidal cavities, 234 is in the range of 0 to 20 um. The template nowcomprises a staggered pattern made of large inverted pyramidal cavity236 and an adjacent small inverted pyramidal cavity 238. The anglebetween the cavity sidewalls and top surface ridges aligned along the(100) crystallographic plane, the lateral plane, is 54.7°.

FIGS. 6E through 6G depict cross-sectional drawings illustrating aprocess flow for manufacturing a 3-D TFSS using an inverted pyramidalsilicon template.

As shown is FIG. 6E, porous silicon layer 244 is formed byelectrochemical HF etching on the front surface of silicon template 242.The porous silicon is used as a sacrificial layer and may consist of twothin layers with different porosities. The first thin porous siliconlayer is on the top and is formed first from silicon wafer 242. Thefirst thin layer preferably has a lower porosity of 10%˜35%. The secondthin porous silicon layer is formed directly from silicon wafer 242 andis underneath the first thin layer of porous silicon. The second thinporous silicon layer preferably has a higher porosity in the range of40%˜80%. The lower porosity porous silicon layer on top is used as acrystalline seed layer for high quality epitaxial silicon growth and theunderneath higher porosity porous silicon layer is used for facilitatingTFSS releasing due to its less density physical connections between theepitaxial and bulk silicon interfaces and its weak mechanical strength.Alternatively, a single porosity release layer with a progressivelyincreased or graded porosity from top to bottom may also be used. Inthis case, the top portion of the porous silicon layer has a lowporosity of 10% to 35% and the lower portion of the porous silicon layerhas a high porosity of 40% to 80%.

FIG. 6F illustrates silicon template 252 after a thin layer of epitaxialsilicon layer growth. In a batch process, after short hydrogen annealingin a temperature range of 950° C. to 1150° C., mono-crystalline siliconepitaxial layer 256 is deposited on porous silicon layer 254 located onthe front side of silicon template 252. Mono-crystalline siliconepitaxial layer 256 may p-type, boron (B₂H₆) doped during the growth.The thickness of the epitaxial layer is preferably in the range of 5 umto 60 um. It is known that crystallographic orientation is one of thefactors that affect the epitaxial growth rate. In the presence of a(100) and a (111) plane on the template, the epitaxial growth rate onthe (100) plane is faster than on the (111) plane. The growth ratedifference could be as large as 20%. Since the template ridge topsurface is a (100) plane and the pyramid cavity sidewalls are (111)planes, the epitaxial silicon layer thickness at the top ridge region258 is generally thicker than the sidewall regions 259. In addition,since the template top ridge surfaces are more exposed to the gas flowthan the wall surfaces defining the inverted pyramidal cavities duringthe epitaxial growth process, the top portions (forming the baseopenings of the inverted pyramidal cavities) of the epitaxial layer ofthe pyramid structure are thicker than the bottom portions (forming thewalls defining the inverted pyramidal cavities). This gas transportationlimited growth rate differential could be enhanced by tuning gaspressures, flow rates, chamber sizes, and other physical parameters ofthe epitaxial process. Furthermore, the higher epitaxial growth rates attop portions of the pyramid cavities also generate faceting around theridge areas. The faceting effect may changes the square opening patternsinto polygon opening patterns as shown in FIG. 4A. The combinedthickness increases (overgrowth) and shape changes generate a uniquestructure that resembles a prism-grid structure. As a result, the 3-DTFSS of the present disclosure provides the following unique features:

(1) The thickness increase and resulting polygon shape formed on the topsurface of a 3-D TFSS provides significant enhancement to its mechanicalrigidity and strength. The template top ridges correspond to theV-grooves of 3-D TFSS when viewed from the backside. When a 3-D TFSS isunder a bending load, the V-groove areas have higher stressconcentration than the sidewall areas. The increased thickness and thepolygon shape at the top portion therefore enhance the mechanicalhandle-ability of the 3-D TFSS;

(2) The polygon shape and the faceting at its edges and corners providebetter light-trapping effects than a square shaped pyramid structure;

(3) After a certain amount of epitaxial growth from the pyramidtemplate, the top surface profile at the ridges may be sharpener thanthe original template ridge surface profile. The top surface ridgesharpening effect may increase the optical trapping and/or electricalefficiencies; and

(4) After the epitaxial growth from the pyramid template, the TFSSsurfaces are made of crystallographic planes. When the top surface ofthe epitaxial layer (before or after the 3-D TFSS release) is exposed toa diluted anisotropic etchant, such as KOH, for a short time, the topsurfaces can be further sharpened to increase the optical and electricalefficiencies of the resulting solar cells. Thus, the disclosed subjectmatter takes advantage of the higher etch rates of convexcrystallographic edges than concave edges in anisotropic siliconetching.

FIG. 6G illustrates 3-D TFSS 264 that is released from silicon template262. Prior to the release, an encompassing border trench, not shown inthe figure, may be made on the peripheral of the active wafer area tofacilitate the release. The encompassing trenches are formed bycontrolled laser cutting and their depths are preferably in the range of5 um to 100 um. The trenches define the boundary of the 3-D TFSS to bereleased and allow initiation of the release from the trenched region.Alternatively, the thin epitaxial layer on the template edge could beremoved first by mechanical grinding and then defining the shape of the3-D TFSS by laser trimming after it has been released from the template.The released epitaxial layer, referred to as 3-D TFSS 264 is physicallyseparate from silicon template 262. The epitaxial layer release methodsdisclosed in U.S. patent application Ser. No. 12/473,811 entitled,SUBSTRATE RELEASE METHODS AND APPARATUS are hereby incorporated byreference. The epitaxial layer may be released in an ultrasonic DI-waterbath. In another release method, the epitaxial layer is released bydirect pulling with wafer backside and top epitaxial vacuum chucked.Using this method the porous silicon layer may be fully or partiallyfractured. The chucks may use either electrostatic or vacuum chucking tosecure the wafer. The wafer is first placed on bottom wafer chuck withTFSS substrate facing upwards. A bottom chuck secures the template sideof wafer, and the top wafer chuck is gently lowered and secures TFSSsubstrate side of the wafer. The activated pulling mechanism lifts topchuck upwards, and the movement may be guided evenly by slider rails.

After removal of the epitaxial silicon layer from the template, thetemplate is cleaned by using diluted HF and diluted wet silicon etchsolution, such as TMAH and/or KOH to remove the remaining porous siliconlayers and silicon particles. The template may then be further cleanedby conventional silicon wafer cleaning methods, such as SC1 and/or SC2wet cleaning to removal possible organic and metallic contaminations.Finally, after proper rinsing with DI water and drying, the template isready for another re-use cycle. Next, the released TFSS backside surfaceis cleaned by short silicon etching using KOH or TMAH solutions toremove the silicon debris and fully or partially remove the QMS layer.

One of the key factors in the template structural design is the use ofinverted pyramidal cavity structures instead of non-inverted pyramidalpillar structures. In the present disclosure, the corners/edges where(111) planes meet are “concave”. In other words, the (111) planes formthe sidewalls of pyramidal cavities. In comparison, there have beenreported “convex” cases, where the (111) planes form the sidewalls ofpyramidal pillars. An inverted-pyramid cavity structure with “concave”corners is preferable over a non-inverted pyramid pillar structure with“convex” corners because of the following reasons:

(1) The silicon anisotropic etching of inverted-pyramidal cavitiesself-terminates when two (111) plane meet, while in thenon-inverted-pyramid pillars case, the etching continues with higheretch rate at the convex edges where two (111) planes meet. Therefore,from the manufacturability perspective, the former case is preferredbecause of its convenient process control;

(2) Because inverted pyramidal cavities have only (111) planes forsidewall and (100) planes for top surface, an epitaxial growth fromthese crystallographic surfaces have better geometry and process controlthan the non-inverted-pyramidal pillar case;

(3) Because the inverted pyramidal cavities consist of concave (111)plane intersections, the silicon template can be conveniently cleanedand re-conditioned in a short time by anisotropic etching after eachre-use cycle or once every several reuse cycles.

The mechanical handle-ability of the 3-D TFSS is another key factor inthe template structure design.

FIG. 7 illustrates a template having an array, or non-staggered, patterninverted pyramidal cavities. Pyramidal cavities 302 all have the samebase opening size and thus the same cavity depth, and are arranged in anarray. Ridges 304 form lateral rows and columns between the invertedpyramidal cavities. These ridges may be referred to as frames, grids,space lines, or ridge lines. Ridges 304 are aligned to the (100)crystallographic direction of the template. The straight long ridgesbetween the cavities on the template will be transferred to the backsideof a corresponding 3-D TFSS made in accordance with the disclosedsubject matter in long V-groove shapes. The V-grooves on the 3-D TFSSare aligned to the (100) crystallographic directions. Thus when the 3-DTFSS experiences in-plane or out-of-plane bending or twisting, stressconcentration and bending moment on the long V-grooves are higher thanthe inverted pyramidal cavity sidewalls. Therefore, the V-grooves behavelike an out-of-plane bending/rotation axis, resulting in a very flexiblesubstrate. Additionally, because the V-grooves are aligned in the <100>direction, once a micro fracturing is initiated from either the edge orthe middle of the 3-D TFSS, it propagates easily along the V-grooves andcauses the TFSS to crack. As a result, an advantage of this type of TFSSis mechanical flexibility and a disadvantage is that the TFSS isrelatively weak. Therefore one of the key factors in making TFSS withreliable mechanical rigidity and strength is to avoid long and straightridges, shown as ridges 304, on the template.

FIG. 8A through 8D illustrate four examples of template layout patternsof staggered inverted pyramidal cavity designs. An advantage of oneembodiment of the present disclosure to increase the mechanical strengthof a 3-D TFSS through staggered patterns of inverted pyramidal cavities.Staggered patterns avoid long V-groove on the 3-D TFSS because staggeredpatterns limit the length of the ridges forming the base openings of theinverted pyramidal cavities (in both rows and columns as shown form atop view of the template). The staggered pattern designs of the presentdisclosure are not limited to the described embodiments but insteadinclude any staggered pattern of inverted pyramidal cavities.

FIG. 8A shows a staggered inverted pyramidal cavity layout pattern thatconsists of two cavity sizes. Small cavity 324 has a base opening halfthe size of the base opening of large cavity 322. In this case, theV-groove length on the corresponding 3-D TFSS will be about 1.5 times aslong as the length of one side of the base opening of large cavity 322.Additionally, each ridge on template 320 and each V-groove on the formed3-D TFSS is intersected at twice (each at one third the length of theridge or V-groove) by neighboring perpendicular ridges or V-grooves. Asa result, the intersection of V-grooves of a 3-D TFSS made with thisstaggered pattern are uniformly spread giving this staggered patterngood mechanical handle-ability and uniform mechanical strength acrossits lateral plane.

FIG. 8B presents another alternative staggered inverted pyramidal cavitylayout pattern that consists of two cavity sizes. Small cavity 344 has abase opening that is a fraction of the base opening of large cavity 342.Shown, the ratio between the two cavity sizes is between 1 and 2. Thiscavity layout pattern is a general pattern of the layout shown in FIG.8A. In this design, the V-groove length on the formed 3-D TFSS is about1 to 2 times of the cavity length. Furthermore, alternatively theinverted pyramidal cavity layouts may include different invertedpyramidal cavity shapes (such as rectangular) and staggered invertedpyramidal cavities with more than two sizes.

FIG. 8C presents an alternative staggered inverted pyramidal cavitylayout that consists of rectangular cavity 362 and square cavity 364.Square cavity 364 has a base opening size equal to the width of the baseopening of rectangular cavity 362. Each square cavity is surrounded bytwo pairs of perpendicularly arranged rectangular cavities. Thus, thelength of the V-grooves on the formed 3-D TFSS will be about the lengthof the base opening of rectangular cavity 362 plus twice the width ofrectangular cavity 362.

FIG. 8D presents yet another alternative staggered inverted pyramidalcavity layout that consists of one size of inverted pyramidal cavities.Each rectangular cavity 382 has the same size and is arranged in astaggered perpendicular format. In this case, the length of theV-grooves on the 3-D TFSS will be about the length of the base openingof rectangular cavity 382 plus the width of rectangular cavity 382 onthe template.

FIG. 9 is a process flow depicting major fabrication process steps of anexemplary method for making a 3-D TFSC using the released 3-D TFSS, in ablock diagram 400. The 3-D TFSC fabrication process starts in Step 405with a p-type silicon 3-D thin film substrate (3-D TFSS) having invertedpyramidal ridges on a top surface plane and inverted pyramidal apexregions on a bottom surface. Step 410 involves selectively coating thetop ridge areas of the 3-D TFSS with an n-type (such as phosphorus)liquid dopant. Viewed from a top perspective, the coated areas form longlines that are connected at cell edges to form fingers and busbarspatterns as in regular flat silicon based solar cells. However, thedoped fingers and busbar lines on the top ridge areas may not bestraight lines if the inverted pyramidal cavity pattern layout isstaggered. The selective liquid dopant coating may be done by alignedscreen printing, roller coating, or direct inkjet dispense. After thecoating, the coated layer is dried and cured (e.g., by thermal curing at250° C. to 400° C. or UV irradiation). Step 415 involves selectivelycoating the bottom side of the 3-D TFSS with p-type (such as boron)liquid dopant. The liquid dopant is selectively coated to the invertedpyramidal apex regions on the bottom surface of the 3-D TFSS by alignedor self-aligned roller coating, screen printing, or dip-coating methods.After the coating, the coated layer is dried and cured (e.g., by thermalcuring at 250° C. to 400° C. or UV irradiation). Step 420 involvesforming n++ selective emitter and p++ base diffusion contact regions bythermal annealing that may be done in a diffusion furnace at 800° C. to950° C., where the emitter and base are concurrently formed. Step 425involves a surface passivation process. In one embodiment, a thermaloxide layer of 10 to 200 nanometers is grown at 800° C. to 950° C. Inanother embodiment, PECVD silicon nitride thin layer could also be usedas a surface passivation layer. The surface passivation layers areapplied on both the top and bottom surfaces of the 3-D TFSS. Step 430involves making local openings on the emitter and base contact regionsby selective passivation layer chemical etching, such as by applyingHF-based etchant by inkjet dispensing. The contact openings are madesmaller than the dopant diffused areas to avoid shunting aftermetallization. Step 435 involves self-aligned metallization. The emitterand base metallized regions are concurrently formed using selectiveelectroplating and/or electroless plating to form single or multilayerhigh-conductivity metallized regions of silver, aluminum, nickel,titanium, cobalt, or tantalum. For instance, the plated metal stack mayinclude a thin (50 to 500 nanometers) barrier and adhesion layer (madeof nickel) followed by a relatively thick (2 to 15 microns) layer ofhigh-conductivity metal (silver, copper, or aluminum). In anotherembodiment, the metal contacts may be formed by the aligned inkjetdispense or screen printing of metal particles, such as silvernano-particles in a liquid solution or paste. Step 445 involves mountingthe 3-D thin film solar cell (3-D TFSC) onto a plate with a metalsurface or metal lines to interconnect the base contacts. The metalplate preferably has a reflective surface to serve as a rear reflectionmirror. The mounting could direct metal-to-metal fusion or with a highlyconductive adhesive. Step 450 involves packaging the fabricated solarcell into a solar module assembly. In this manufacturing module, theemitter and base metal contacts are interconnected among the solar cellsto form the power output connections of a solar panel.

FIG. 10A through 10D illustrate partial cross-sectional views of aprocess flow for manufacturing a three-dimensional thin-film solar cellaccording to the process steps of FIG. 9.

FIG. 10A illustrates 3-D TFSS 512 after the selective emitter 514 andbase 516 coating steps. The liquid dopants, such as phosphorus-containedliquid for emitter and boron-contained liquid for base, are dried andcured after their selective coatings.

FIG. 10B illustrates 3-D TFSS 522 after the selective emitter 524 andbase 526 diffusion and passivation layer 528 coating steps. The emitterand base diffusion regions are concurrently formed in a diffusionfurnace with a controlled time and temperature. The actual dopingprofile may be extended towards to the sidewalls near the contactregions. After the emitter and base diffusion, the remaining dopantmaterial and dielectric layers formed during the diffusion process areremoved. A passivation layer is then applied on both the front and basesurfaces of the 3-D TFSS. Examples of the passivation layer includethermally grown silicon dioxide and PECVD silicon nitride.

FIG. 10C illustrates 3-D TFSS 532 after the selective emitter 534 andbase 536 contact openings are formed. The local openings on the emitterand base contact regions are made by selective passivation layerchemical etching, such as by applying HF-based etchant by inkjetdispensing. The contact openings are made smaller than the dopantdiffused areas to avoid shunting after metallization. Portions of thetop and bottom surface of the 3-D TFSS remain coated with passivationlayer 538.

FIG. 10D illustrates a completed 3-D TFSS 542 after all the cellfabrication process disclosed in FIG. 9. The emitter metal 544 and basemetal 546 are electroplated or electroless plated single or multilayerhigh-conductivity metallized regions (silver, aluminum, nickel,titanium, cobalt, or tantalum). Alternatively, the metal layer could beinkjet dispensed. The emitter metal contacts are formed in continuousmetal lines, i.e. fingers and busbars on the 3-D TFSC top surface.However, because the base metal contacts have been formed on theinverted pyramidal apex regions on the backside of the 3-D TFSS, thebase metal contacts are isolated regions. It is to be noted, the frontsurface passivation layer may also serve as the antireflection layergiven a controlled thickness. In one embodiment, the 3-D TFSS aftermetallization is mounted on a supporting non-metal plate 550 withcontinuous metal surface or patterned metal lines 552 to form the baseinterconnects of the 3-D TFSC. Portions of the top and bottom surface ofthe 3-D TFSS remain coated with passivation layer 548.

The 3-D TFSS and cell process flows as shown is FIGS. 5, 6, 9 and 10 maybe applied to substrate doping polarity of n-type for p-type selectiveemitters. Thus the 3-D TFSS base could be either n-type or p-type withcorresponding emitter polarities.

The foregoing description of the preferred embodiments is provided toenable any person skilled in the art to make or use the claimed subjectmatter. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinnovative faculty. Thus, the claimed subject matter is not intended tobe limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

1. A method for fabrication of a structure for use in a photovoltaicsolar cell, the method comprising: selectively removing material from amonocrystalline silicon wafer to form a reusable monocrystalline silicontemplate having a top surface aligned along a (100) crystallographicplane of said template and a plurality of walls each aligned along a(111) crystallographic plane of said template, said walls defining aplurality of inverted pyramidal cavities comprising a set of largerinverted pyramidal cavities with an opening size in the range ofapproximately 10 μm up to 1 mm and a set of smaller inverted pyramidalcavities with an opening size a fraction of said opening size of saidlarger inverted pyramidal cavities, said set of larger pyramidalcavities and said set of smaller inverted pyramidal cavities arranged ina staggered pattern on said template which limits the length of ridgesof said pyramidal cavity openings to less than twice the width of theopenings of said larger pyramidal cavities; forming a substantiallyconformal sacrificial porous silicon layer comprising at least twodifferent porosities coupled to said template; and epitaxiallydepositing a monocrystalline silicon layer to form a thin-film solarcell substrate on said sacrificial porous silicon layer, said solar cellsubstrate having a surface topography defined by said inverted pyramidalcavities on said reusable template; said inverted pyramidal cavities onsaid reusable template ensuring that said solar cell substrate iscapable of being detached from said template as a free-standing,self-supporting substrate with sufficient mechanical rigidity for use ina manufacturing environment.
 2. The method of claim 1, wherein said stepof selectively removing silicon material further comprisesanisotropically etching the silicon template to form a plurality ofwalls each aligned along a (111) crystallographic plane of the silicontemplate wherein said walls form an inverted pyramidal cavity.
 3. Themethod of claim 2, wherein said step of anisotropically etching thesilicon template utilizes KOH or NaOH as an etchant.
 4. The method ofclaim 2, wherein said step of anisotropically etching the silicontemplate utilizes TetraMethyl-Ammonium-Hydroxide (TMAH) as an etchant.5. The method of claim 1, wherein said step of selectively removingsilicon material further comprises: forming a hard masking layer on thesurface of the silicon template; depositing a patterned soft maskinglayer on said hard masking layer on selected surface areas of thesilicon substrate to be preserved as top surfaces; etching said exposedhard masking layer; removing said patterned soft masking layer exposingsaid hard masking layer underneath; anisotropically etching exposedsurfaces of the silicon template to form a plurality of walls eachaligned along a (111) crystallographic plane of the silicon templatewherein said walls form an inverted pyramidal cavity; and selectivelyremoving the remaining patterned hard masking layer.